Method and device to visualize in-line and quantify the polymer melting in plasticating screw machines without significantly affecting its thermal regime

ABSTRACT

A device to visualize in-line the polymer melting in plasticating screw machines has an external metallic cylinder to be fixed in radial position regarding the barrel of the plasticating machine, a bushing with an optical window clear to the visible light spectrum resistant to pressure and temperature positioned in such a way that its external face is tangent to the internal surface of the barrel of the plasticating machine, and an internal metallic cylinder to guide the coolant gas and allow the use of an observation probe with operating temperature limits and an illumination system.

SUMMARY

The invention comprises a method and a device to visualize in-line andquantify the polymer melting in plasticating screw machines duringprocesses that may include phase changes of the polymeric material understudy, such as extrusion, blow molding and injection molding. Theinvention is based on the need to visualize and quantify the polymermelting in plasticating screw machines in real time, safely andnon-invasive to the process to be able to maintain its thermal regime.The problem is solved by capturing images in-line and experimental datafrom the inside of the plasticating screw machine through a device witha small optical window, which allows the use of an observation probewith operating temperature limits and with an illumination system. Thedevice allows to visualize the differences in optical properties betweenthe polymer's solid state and its melt state enabling to describe itscomplete melting mechanism in a plasticating screw machine. The deviceallows to visualize all of the states or phases of the polymer meltingprocess, such as the solid material, the formation of the first meltfilm, the delay in the formation of the melt pool, the molten materialand anomalies in the melting, in case these latter exist, since severalpolymers do not present them. The device is comprised of an externalmetallic cylinder to be set in radial position regarding the barrel ofthe plasticating machine, a bushing with an optical window clear to thevisible light spectrum, resistant to the pressure and to thetemperature, positioned in such a manner that its external face istangent to the internal surface of the plasticating machine's barrel,and an internal metallic cylinder to guide the coolant gas and enablethe use of an observation probe with operating temperature limits andwith an illumination system. The method to quantify the polymer meltingin plasticating screw machines comprises the following stages:

-   -   Location of several devices according to claims 1 to 10 along        the barrel of the plasticating screw machine.    -   In-line visualization of the polymer inside the machine either        in solid, melt state or in co-existing states, through an        observation probe placed in the device of claims 1 to 10.    -   Joint calibration of the device's optical window according to        claims 1 to 10 and of the observation probe to determine the        relationship between the visualized image and the quantified        image in pixels, and thus, guarantee the precision of the        measurements from the observed images.    -   Recording of the polymer's plasticating process by capturing the        images in-line and the experimental data from the inside of the        plasticating screw machine with a camera and a video recorder.    -   Determination of the solids bed's position and width from the        obtained images, operating conditions and plasticating machine's        geometry using an image analyzer program or software.    -   Calculation of the polymer's melting profile visualized in the        plasticating screw machine.

DESCRIPTION

The invention comprises a method and a device to visualize in-line andquantify polymer melting in plasticating screw machines during processesthat can include phase changes of the polymeric material under study,such as extrusion, blow molding and injection molding.

As it is widely known in the scientific and technical community, theexisting experimental techniques designed to study the melting ofpolymers inside a plasticating screw machine suffer a very limitedfunctionality and very tedious procedures. The techniques developed inthe last decades are invasive to the process, highly time consuming,difficult to implement, and expensive. The invasive nature affects thepolymer's characteristics of friction and heat transfer inside theplasticating screw machine, significantly influencing the measuredparameters and consequently, the results, particularly the polymer'smelting profile.

The first experimental method to analyze polymer melting in plasticatingscrew machines was developed by Maddock and Street, and consisted inabruptly stopping an extruder in operation and cooling both the barreland the screw, and then remove the screw from the barrel, remove thepolymer from its spirals and cut it in perpendicular sections to thescrew's flights. To visualize details of the melting process with thistechnique, they added a small amount of pigmented polymer or tracer,which made it possible to distinguish the polymer's regions in solidstate and in melt state. This technique is known as the extractiontechnique, which is very intensive in time consumption, requirements forthe experimentation and costs. Tadmor developed a theoretical modelbased on Maddok's experimentation methodology to predict the profile ofthe polymer melting in plasticating single screw machines. Based on thismodel, other authors have developed similar models for plasticatingmachines with more than one screw. The most recently developedexperimental method, towards the end of the 90's, to analyze the polymermelting in plasticating screw machines was developed by Wong, Zhu andcollaborators based on an extruder with a barrel equipped with largeglass windows on both sides of the barrel. Important results wereobtained with this technique, but it resulted invasive to the processand affected the polymer's heat transfer within the plasticating screwmachine and thus, negatively influencing the measured parameters andconsequently, the results. Additionally, this method presented a riskfor the extruder's operator inherent to the rupture of the large glasswindows.

The general objective of the invention herein is to visualize andquantify the polymer melting in plasticating screw machines in realtime, safely and non-invasive to the process, to be able to maintain itsthermal regime providing a method and a device according to claims 1 to12. The problem is solved by capturing images in-line and experimentaldata from the inside of the plasticating screw machine through a devicewith a small optical window, which allows the use of an observationprobe with operating temperature limits and with an illumination system.The device allows to visualize the differences in optical propertiesbetween the solid state and the melt state of a polymer allowing todescribe its complete melting mechanism in a plasticating screw machine.

To clarify the invention even more and its advantages compared with theexisting techniques, the device and the method are described with thehelp of the enclosed drawings.

In figures:

FIG. 1 schematically shows a cross-section of the barrel of aplasticating screw machine, which points out the device radially fixedon it and tangent to the barrel's internal surface.

FIG. 2 shows a perspective representation of the device to visualizein-line the polymer melting in plasticating screw machines.

FIG. 3 schematically shows a view of the device with all of itsassembled components.

FIG. 4 schematically shows the external metallic cylinder of the devicewith its connections for the inlet and outlet of the coolant gas.

FIG. 5 schematically shows the internal metallic cylinder of the device,which guides the coolant gas and receives inside the observation probe(omitted in the figure).

FIG. 6 a shows as an example of the invention of an image captured fromthe inside of the plasticating screw machine, whereby one can visualizethe polymer's solid state, the polymer's melt state and the screw'sflight.

FIG. 6 b shows as an example of the invention an internal schematic ofthe plasticating screw machine, whereby it can be identified the solidstate of the polymer, the melt state of the polymer and the screw'sflight.

FIG. 7 schematically shows a cross-section of the barrel of another typeof plasticating machine with more than one screw, where it is possibleto observe the device fixed to it and tangent to the barrel's internalsurface.

FIG. 8 shows the location of several devices along the barrel of theplasticating screw machine as a fundamental arrangement for the methodto quantify the polymer melting in plasticating screw machines.

FIG. 9 shows a melting profile of a polymer obtained from a plasticatingscrew machine with several devices according to claims 1 to 10.

With reference to the figures as can be seen in FIG. 1, a barrel of aplasticating screw machine generically indicated as 1 has a devicescrewed on or fixed indicated as 2 radially and tangent to the barrel'sinternal surface indicated as 3 of the plasticating machine.

On FIG. 2 a perspective representation can be observed of the device andFIG. 3 shows an external metallic cylinder indicated as 4 to be screwedon or radially fixed with respect to the barrel of the plasticatingmachine 1, a bushing 7 with a clear optical window 8 to the spectrum ofvisible light resistant to the pressure and temperature, positioned suchas that its external face 9 is tangent to the internal surface 3 of thebarrel of the plasticating screw machine, and an internal metalliccylinder 5 to guide the coolant gas and enable the use of an observationprobe (omitted in the figure) with operating temperature limits and withan illumination system. FIG. 4 illustrates the external metalliccylinder indicated as 4 with its connections 6 for the inlet and outletof coolant gas. The internal metallic cylinder 5 is screwed on or fixedinside the external metallic cylinder 4. FIG. 5 illustrates the internalmetallic cylinder 5, which possesses on its surface a spiral, baffles ordeflectors 10 to guide the coolant gas throughout its length. Thisinternal metallic cylinder receives inside the observation probe(omitted in the figure) with operating temperature limits and with anillumination system. FIG. 6 a and FIG. 6 b illustrate, as a form ofcarrying out the invention, an image captured from the inside of theplasticating screw machine and a scheme from where it is possible tovisualize the polymer's solid state 11, the polymer's melt state 12, andthe screw's flight 13. Clear differences are observed between thepolymer's solid state and melt state due to the differences in densityand the optical properties of each state, allowing to describe thepolymer's complete melting mechanism in the plasticating screw machine.FIG. 7 shows, as a form of carrying out the invention, a barrel ofanother type of plasticating machine with more than one screwgenerically indicated by 1 that has a device screwed on or fixed,indicated by 2, and tangent to the barrel's internal surface, indicatedby 3 of the plasticating machine. FIG. 8 shows as a form of carrying outthe invention, the location of several devices 2 along the length of thebarrel of the plasticating screw machine 1 as a fundamental arrangementfor the quantification method of the polymer melting in plasticatingmachines. This arrangement allows to visualize in-line the developmentof the polymer's melting inside the plasticating machine through theobservation probe (omitted in the figures) with an illumination systemand to capture images with a camera and video recorder. A jointcalibration is carried out of the optical window 8 of the device 2 andof the observation probe (omitted in the figures) to determine therelationship between the visualized image and the quantified image inpixels, and thus, guarantee the precision of the measurements based onthe observed images. The images allow to determine the position andwidth of the polymer's solids bed X, the operating conditions, and theplasticating machine's geometry using an image analyzer program orsoftware. In the case of a plasticating machine with only one screw, Xcan be additionally verified with the formula: X=Vb.t.cos(90-φ), where,Vb is the barrel velocity with respect to the screw, t is the timeelapsed in the visualization based on the images between the screw'sflank and the finalization of the solids bed, excluding the flight'swidth, and φ is the screw's helix angle. With X measured, it is thenpossible to calculate the polymer's melting profile inside theplasticating screw machine. The plasticating profile is represented by agraph of the ratio solids bed width X over screw channel width W versusthe ratio of plasticating machine's length L and diameter D. The valuesX/W range between 0 and 1. When X/W=1, the polymer is in the solidstate, and when X/W=0, the polymer is in the melt state. X values arecaptured from the images for different positions in the barrel of theplasticating machine L, where the devices are indicated by 2. Thearrangement is shown as an example in FIG. 8 for a single screwplasticating machine. Afterwards, the polymer's melting profile ischarted. FIG. 9 shows the melting profile of a polymer obtained from aplasticating screw machine with several devices as shown FIG. 8. In thecase of a single screw plasticating machine or a single screw extruder,the theoretical plasticating profile can be predicted according toTadmor's model, whose main equations are detailed below:${X/W} = {X_{0}/{W\left\lbrack {1 - \frac{\psi\left( {z - z_{o}} \right)}{2h}} \right\rbrack}^{2}}$z = L/sin (ϕ) $\psi = \frac{\Phi}{v_{sz}\rho_{solid}\sqrt{Xo}}$$\Phi = \left\{ \frac{v_{bx}\rho_{m}{U_{2}\left\lbrack {{k_{m}\left( {{Tb} - {Tm}} \right)} + {U_{1}/2}} \right\rbrack}}{2\left\lbrack {{C_{s}\left( {{Tm} - {To}} \right)} + {C_{m}{\Theta\left( {{Tb} - {Tm}} \right)}} + \lambda} \right.} \right\}^{1/2}$$\delta = \left\{ {\frac{\left\lbrack {{2{k_{m}\left( {{Tb} - {Tm}} \right)}} + U_{1}} \right\rbrack}{v_{bx}U_{2}{\rho_{m}\left\lbrack {{C_{s}\left( {{Tm} - {Tso}} \right)} + {C_{m}{\Theta\left( {{Tb} - {Tm}} \right)}} + \lambda} \right.}}X} \right\}^{1/2}$Where,

-   -   D: Diameter of the single screw extruder    -   L/D: Dimensionless length of the single screw extruder    -   h: Screw channel depth    -   W: Screw channel width    -   X: Solids bed width    -   X_(o): Initial solids bed width    -   ψ: Dimensionless volumetric flow rate    -   z: Position in channel's direction    -   z_(o): Initial position in channel's direction    -   φ: Screw helix angle    -   Φ: Melting rate    -   V_(bx): Barrel velocity in x direction    -   V_(sz): Velocity of the solids bed in z direction    -   ρ_(solid): Density of the solid polymer    -   ρ_(m): Density of the melt    -   U₁, U₂: Parameters of Tadmor model    -   k_(m): Thermal conductivity of the melt    -   Tb: Barrel temperature    -   Tm: Melting temperature    -   Tso or To: Initial temperature of the solids    -   C_(s): Specific heat of the solid polymer    -   C_(m): Specific heat of the melt    -   Θ: Mean dimensionless temperature    -   γ: Heat of fusion    -   δ: Melt film thickness

On FIG. 9, the measured points are indicated by 14 and the predictionsof the theoretical models are indicated by 15 non-Newtonian model and 16Newtonian model; these names are due to the consideration of thepolymer's viscosity. Both considerations were modeled by Tadmor.

Obviously, the above gathered description of forms of carrying out theinvention that apply the innovative principles of the herein inventionare given as examples, and consequently, should not be taken as alimitation of the claims.

1. Device to visualize in-line the polymer melting in plasticating screwmachines, which comprises an external metallic cylinder to be fixed inradial position regarding the barrel of the plasticating machine, abushing with an optical window clear to the visible light spectrumresistant to pressure and temperature, positioned in such a way that itsexternal face is tangent to the internal surface of the barrel of theplasticating machine, and an internal metallic cylinder to guide thecoolant gas and allow the use of an observation probe with operatingtemperature limits and an illumination system.
 2. Device according toclaim 1 to be screwed on or installed in plasticating screw machines,such as single screw extruders or extruders with more than one screw,blow molding and injection molding machines.
 3. Device according toclaim 1 characterized because the transparent optical window ismanufactured of quartz, glass or sapphire with a maximum thermalresistance up to 1000° C.
 4. Device according to claim 1, characterizedbecause the optical window is small, round and has a diameter equal toor less than 10 mm to not significantly affect the thermal regime of theplasticating screw machine.
 5. Device according to claim 1,characterized because the optical window has a minimum thickness of 6 mmto ensure the pressure resistance existing inside the plasticating screwmachine.
 6. Device according to claim 1 characterized because theoptical window can be replaced in case of damage.
 7. Device according toclaim 1 characterized because the bushing with the optical window is aring made of a deformable material and self-sealant under temperatureand pressure.
 8. Device according to claim 1 characterized because thebushing is a thermoplastic material that does not melt at the operatingtemperature of the plasticating machine and is deformable to guaranteethe self-sealant effect.
 9. Device according to claim 1 characterizedbecause the internal metallic cylinder possesses on its surface aspiral, baffles or deflectors to guide the coolant gas throughout itslength, to have good heat removal and preserve the observation probewith operating temperature limits and with illumination system. 10.Device according to claim 1 characterized because the external metalliccylinder possesses two connections, one for the inlet and another forthe outlet of the coolant gas.
 11. Method to quantify the polymermelting in plasticating screw machines through the device of claim 1,which is characterized due to the in-line visualization of the process,in real time, safely and not significantly affecting its thermal regime.12. The method to quantify the polymer melting in a plasticating screwmachine characterized by the following stages: location of severaldevices according to claim 11 along the length of the barrel of theplasticating screw machine; in-line visualization of the polymer insidethe machine, either in solid, melt state or in co-existing states,through an observation probes placed in the devices; joint calibrationof the device's optical windows and of the observation probe todetermine the relationship between the visualized image and thequantified image in pixels, and thus, guarantee the precision of themeasurements from the observed images; recording of the polymer'splasticating process by capturing the images in-line and theexperimental data from the inside of the plasticating screw machine witha camera and a video recorder; determination of the solids bed'sposition and width from the obtained images, operating conditions andplasticating machine's geometry using an image analyzer program orsoftware; and calculation of the polymer's melting profile visualized inthe plasticating screw machine.